Judging from the digestion ability of a soldering furnace for continuous board feeding in a certain reflow zone, it is obvious that the furnace temperature is continuously decreasing, and the thermal compensation ability is fundamentally insufficient, and the maximum drop is 4～5℃. And this kind of temperature drop does not only occur in a certain reflow zone. All 10 heating zones have a temperature drop of 4 to 8°C, which is the root cause of occasional cold welding.
Furthermore, we conducted investigations into the period of continuous and frequent board loading. This period happened to be during the day-night shift. A batch of boards were stacked in front of the furnace. In order to increase the production capacity, the boards were manually pushed into the furnace. The placement machine can enter the working state as soon as possible.This phenomenon is strictly prohibited in production management.
The case has now come to an end, but have we ever considered a question: There will be fluctuations in the welding furnace, and it is difficult to avoid human intervention in the interval of entering the board, so is there another suitable solution? Let's share the process risks and avoidance methods together.
1) Is the furnace process robust enough?
Many users order the start of production as long as the temperature curve results are qualified when using a thermometer to measure the furnace temperature. Is this process robust? We will be clear by comparing the following two examples!
(Figure 6) Process A: PPI process risk warning
(Figure 7) Process B: PPI process is controlled
As shown in Figure 6 and Figure 7, all the curve data are within the range of process specifications, but the process B in Figure 7 is more robust than the process A in Figure 6, how to determine?
First, analyze the PPI (Process Performance Index). In addition to the test results of each parameter of each channel that need to meet the process specifications, where the test results are located within the upper and lower limits of the process specifications is the main factor affecting the robust process. The following will explain the peak temperature of the second channel:
Peak specification: 230℃(PPI=-100)~250℃(PPI=100);
1) If the measured peak value of process A: 230.0℃ (PPI=-100);
2) If the measured peak value of process B: 240.0℃ (PPI=0);
It can be seen that both processes A and B meet the requirements, but process A has reached the lower limit of specifications. A slight temperature fluctuation of the welding furnace or an increase in the frequency of the board will lead to insufficient peak temperature and cold welding.
The B process is not the case. Its ability to resist temperature fluctuations of the soldering furnace and the ability to resist full load is very strong. Even if the board is loaded too fast, there is still room for fluctuations.